cpu: add support for detecting RISC-V extensions

Add a RISCV64 variable to cpu that indicates both the presence
of RISC-V extensions and performance information about the underlying
RISC-V cores.  The variable is only populated with non false values on
Linux.  The detection code first attempts to use the riscv_hwprobe
syscall introduced in Linux 6.4, falling back to HWCAP if riscv_hwprobe
is not supported.  The patch can detect the C, V, Zba, Zbb and Zbs
extensions.  V, Zba, Zbb and Zbs can only be detected on a 6.5 kernel
or later (without backports).

Updates golang/go#61416

Change-Id: I40f92724ee3d337c06bdc559ff0b18a8f6bfda9f
Reviewed-on: https://go-review.googlesource.com/c/sys/+/605815
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Joel Sing <joel@sing.id.au>
Reviewed-by: Meng Zhuo <mengzhuo1203@gmail.com>

cpu/cpu_riscv64.go

@@ -8,4 +8,13 @@ package cpu
 
 const cacheLineSize = 64
 
-func initOptions() {}
+func initOptions() {
+	options = []option{
+		{Name: "fastmisaligned", Feature: &RISCV64.HasFastMisaligned},
+		{Name: "c", Feature: &RISCV64.HasC},
+		{Name: "v", Feature: &RISCV64.HasV},
+		{Name: "zba", Feature: &RISCV64.HasZba},
+		{Name: "zbb", Feature: &RISCV64.HasZbb},
+		{Name: "zbs", Feature: &RISCV64.HasZbs},
+	}
+}